This invention relates in general to a rotor hub for rotary wing aircraft and in particular for a gimbaled hub for a tilt rotor aircraft having the capability of flying as a conventional helicopter and, by tilting the rotors to a forward facing position, can fly as a fixed wing propeller driven aircraft.
In the past, these aircraft with gimbaled hubs have been restricted to the use of three bladed propellers or prop-rotors because of the use of conventional helicopter rotor design having the blade pitch control mechanism external to the rotor hub. A tilt rotor aircraft using gimbaled rotors allows the rotor hubs to be tilted universally about their rotational axes to provide the effect of a blade flap hinge in a conventional helicopter rotor and to assure an appropriate Delta 3 response of the rotor when the rotor is subjected to wind gust inputs or other sudden control inputs by the pilot. The term Delta three will be further defined in the detailed description of the Invention. In the prior art and in this description, the gimbal mounting of the rotor is accomplished by the use of a pair of spherical elastomeric bearings for supporting the rotor hub on the rotor shaft. This teetering ability of the rotor on the spherical bearing serves to provide the flap axis of the rotor blades.
For tilt rotor aircraft, the proper gust response is often designated "flap up--pitch up" where in a conventional helicopter, the proper gust response is "flap up--pitch down". Delta 3 refers to the angle measured from the rotor rotational axis to the blade pitch arm attachment point and measures the offset between the blade pitch arm attachment point and the flap axis of that same blade. Thus, in a conventional helicopter, when a blade tip is raised about its flap axis (flap-up), the blade pitch is reduced (pitch down). In contrast, in a tilt rotor aircraft, when the blade tip is raised about the flap axis (flap up), the blade pitch is increased (pitch up). For convenience, in this description, flap up-pitch down will be designated as positive Delta 3 and flap up-pitch up will be designated as negative Delta 3.
In the prior art, the amount of Delta 3 that could be built into the rotor design was limited because the blade pitch arms were located outside the rotor hub. With more than three blades on the hub, the Delta 3 angle is severely restricted because with the conventional design with the pitch arms external to the hub, the pitch arms interfere witch each other at very low Delta 3 inputs. It is desirable to have more than three blades on a tilt rotor aircraft in order to reduce vibration and noise in transitional and fixed wing operation modes. With the present invention, the number of blades which can be accommodated on the hub which can provide sufficient Delta 3 motion is increased to five, six, seven or even eight blades. This increased number of blades and the resulting smoother flight greatly reduces pilot fatigue as well as reducing vibration induced mechanical failures. In addition, in the case of a civil tilt rotor transport, the additional blades provide a greatly increased level of passenger comfort.